Talk to Iran’s New President. Warily.

THE election of Hassan Rowhani as Iran’s new president has created a sense that there are new possibilities of progress on the nuclear issue; we need to respond, but warily. Iran’s supreme leader, Ayatollah Ali Khamenei, allowed Mr. Rowhani to win the election recognizing that he had run against current Iranian policies that have isolated the country and invited economically disastrous sanctions. But it isn’t clear why Mr. Khamenei allowed such an outcome, and here are some theories that have been proposed:

He believes that Mr. Rowhani’s election could provide a safety valve for the great discontent within Iran.

He believes that Mr. Rowhani, a president with a moderate face, might be able to seek an open-ended agreement on Iran’s nuclear program that would reduce tensions and ease sanctions now, while leaving Iran room for development of nuclear weapons at some point in the future.

He believes that Mr. Rowhani might be able to start talks that would simply serve as a cover while Iran continued its nuclear program.

He wants to rebalance the power relationship among Iran’s leading factions, reconciling their fissures while restoring the relative weight of the clerics vis-à-vis the Revolutionary Guard. Mr. Rowhani is himself a cleric, but also a likely conciliator who might be a bridge between the harder-line clerics and more pragmatic forces.

None of this means there will be a nuclear deal. Even if he were given the power to negotiate, Mr. Rowhani would have to produce a deal the supreme leader would accept. So it is far too early to consider backing off sanctions as a gesture to Mr. Rowhani.

We should, instead, keep in mind that the outside world’s pressure on Iran to change course on its nuclear program may well have produced his election. So it would be foolish to think that lifting the pressure now would improve the chances that he would be allowed to offer us what we need: an agreement, or credible Iranian steps toward one, under which Iran would comply with its international obligations on the nuclear issue. Our bottom line here is that Iran must be prepared to change its program so that it does not have a breakout capability to develop nuclear weapons.

The real question for ourselves is whether we should change our approach to diplomacy with Iran, now that a new Iranian president has advertised his desires to end Iran’s isolation and the sanctions imposed on it, and to repair the “wound” that he has said exists between the United States and the Islamic Republic.

Until now, we have taken an incremental, confidence-building approach within multilateral negotiations with Iran, but they have probably already run their course. Indeed, while our side (the United States, China, Russia, Germany, Britain and France) negotiated with Iran on and off for the last several years with no results, the Iranians were dramatically expanding the numbers of centrifuges they had installed to enrich uranium. They now have roughly 17,000 and have succeeded in upgrading to a new generation of far more efficient centrifuges.

Those developments have shrunk the time we have available to ensure that the Iranians cannot break out and present the world with the fait accompli of a nuclear weapons capability. So we may have time for diplomacy, but not a lot.

We should move now to presenting an endgame proposal — one that focuses on the outcome that we, the United States, can accept on the nuclear issue. And we should do so even if our negotiating partners — particularly the Russians — aren’t prepared to accept such a move, since the clock is ticking. We should give Mr. Rowhani a chance to produce, but the calendar cannot be open-ended.

Diplomacy often boils down to two simple elements: taking away excuses for inaction and providing explanations for a deal that could be struck. On the first point, the Iranians say they don’t know what we will accept in the end. The answer should be that we can accept Iran’s having civil nuclear power but with restrictions that would make the steps to producing nuclear weapons difficult, as well as quickly detectable. Our offer should be credible internationally; if Iran was not prepared to agree to it, the Iranians would be exposed for not being ready to accept what they say they want. Indeed, if we make a credible proposal that would permit the Iranians to have civil nuclear power with restrictions, it would allow them to save face for themselves: they could say the proposal was what they had always sought and that their rights had been recognized.

This is not to say that such an endgame proposal can be made without risk. The Russians, in particular, may not want the situation clarified. They may fear it will mean an end to the diplomacy because the Iranians, in turning down such a proposal, will have signaled that their real aim is to obtain nuclear weapons and not just civil nuclear power. That would leave the use of force as the only alternative. The Russians may prefer the step-by-step approach that keeps the diplomacy going — even without results.

To be sure, if the Iranians were prepared to suspend the further development of their nuclear infrastructure while diplomacy were under way, that would be an acceptable approach and time would not be of the essence. But Mr. Rowhani has already publicly dismissed the possibility of such a suspension, saying it was tried before, but in a different era. So this time, it is the Iranians who are forcing the window for diplomacy to close.

Mr. Rowhani may well create an opening. But we should be on our guard: It must be an opening to clarify what is possible and to test outcomes, not to engage in unending talks for their own sake. Preserving a multilateral step-by-step approach that has outlived its usefulness, or allowing the Russians at this point to determine how we proceed — particularly at a time when the Russians appear more competitive with the United States than cooperative — is not a prescription that permits us to see if there is an opening and to act on it.

If we want diplomacy to succeed, the United States must find out now whether it can, and it must do so on its own initiative.

Dennis B. Ross, a counselor to the Washington Institute for Near East Policy, was the United States’ chief negotiator for the Arab-Israeli conflict from 1993 to 2001 and a special assistant to the president for the Middle East and South Asia from 2009 to 2011.